1. Field of the Invention
The present invention relates to an impurity-doping apparatus, an impurity-doping method, and a semiconductor device manufacturing method.
2. Description of the Related Art
Semiconductor devices using silicon carbide (SiC), especially 4H silicon carbide (4H—SiC), are being expected as power semiconductors. Semiconductor devices of 4H—SiC are typically produced by doping a semiconductor substrate, which includes a 4H—SiC crystalline layer grown epitaxially at a desired concentration, by ion-implantation of impurity elements such as phosphor (P) or aluminum (Al). Specifically, impurity-element ions are accelerated and irradiated onto a semiconductor substrate and implanted into the semiconductor substrate. Then, a process of annealing the semiconductor substrate is performed to recover the crystalline structure of the semiconductor substrate, damaged by the ion-implantation, and activate the impurity elements.
When high-dose amount, for example, about 1015/cm2, of ions are implanted into a (0001) surface ((000-1) surface) of a 4H—SiC semiconductor substrate, it is necessary to heat the semiconductor substrate to about 300 to 800° C. in advance. If the heating is not performed in advance, recrystallization of 4H—SiC and activation of impurity elements are not effectively performed.
The annealing of SiC is performed at about 1600 to 1800° C., which is higher than that of Si. Such high-temperature annealing is known to cause Si atoms to fall off from SiC at the surface of the semiconductor device and roughen the surface of the semiconductor device due to migration. Accordingly, annealing is performed after protection film of aluminum nitride (AlN), carbon (C), or the like is deposed on the surface of the semiconductor device. However, formation of the protection film and its removal increase the number of processes and the processing cost. Moreover, there is a danger of contamination of the surroundings by aluminum (Al) or carbon (C).
As a method for solving such above problems, a technology of laser doping is proposed in non-patent literature (NPTL) 1 and NPTL 2. In NPTL 1 and NPTL 2, the doping method is as follows: a 4H—SiC semiconductor substrate is immersed in a solution as an aqueous solution containing impurity elements, and an interface region between the surface of the semiconductor substrate and the solution is irradiated with laser light. Accordingly, the semiconductor substrate is locally heated and doped by the impurity elements in the solution. The laser lights used in the method proposed by NPTL 1 and NPTL 2 are optical beams having a wavelength in the ultraviolet region, which cause a large absorption coefficient in SiC. According to the techniques of NPTL 1 and NPTL 2, implantation of the impurity elements and activation of the semiconductor substrate are simultaneously performed in an environment substantially at room temperature. Moreover, it is unnecessary to heat the semiconductor substrate in advance and anneal after the implantation of the impurity element.
However, in case of the technology in NPTL 1 and NPTL 2, the semiconductor substrate is needed to be entirely immersed in the solution. Accordingly, a problem arises that a large amount of the solution enough to immerse the semiconductor substrate entirely in the solution is needed for use.